Abstract: Normalization of blood glucose levels via intensive insulin therapy reduces the incidence of diabetic complications. Despite numerous technologic developments such as continuous glucose monitors and closed loop insulin pumps, hypoglycemia unawareness and fear of hypoglycemia remain among the biggest obstacles to achieving tight glycemic control in type 1 diabetic (T1DM) patients. Frequent bouts of hypoglycemia diminish the brain?s capacity to detect hypoglycemia and to activate protective counterregulatory hormonal responses (CRR). As a result hypoglycemia associated autonomic failure (HAAF) with reduced glucagon and epinephrine release increases the risk of more severe hypoglycemic events with adverse consequences including cognitive impairment, seizures and permanent brain injury. This issue is of particular concern in T1DM where recent studies suggest that severe and recurrent hypoglycemia occurring early in a patient?s life can result in cognitive impairment and lasting brain damage. Thus identification of the mechanisms driving counterregulatory failure and central nervous system complications remain an important area of study with the hope of ultimately devising preventive strategies. Previous paradigms have been focused on the contribution of alternate energy substrates such as acetate and lactate to brain metabolism in the context of recurrent hypoglycemia (RH); however in the light of more recent observations, their role appears only limited. Instead, the regulation of brain glucose uptake at the blood brain barrier (BBB) and its neuronal oxidation in mitochondria have emerged as more dominant regulatory steps in this area: We describe for the first time in T1DM how RH exposure limits neuronal glucose utilization by reducing pyruvate dehydrogenase (PDH) activity, thereby providing a rationale for higher lactate production rates. In a recent clinical pilot study we made the exciting observation that pharmacologic re-activation of the PDH complex via the small molecule kinase inhibitor dichloroacetate (DCA) in intensively treated T1DM patients reverses cognitive deficits associated with recurrent hypoglycemia exposure. Under this proposal we will take advantage of a newly developed NRM-based deuterium metabolic imaging (DMI) method that permits metabolism measurements across all areas of the brain simultaneously to determine in a combination of preclinical and clinical studies the mechanism by which DCA affects glucose uptake, oxidative metabolism and regional lactate production and how this ultimately leads to preserved brain energetics, hormonal counterregulation and cognitive function under hypoglycemia. In the end these studies will yield important new information to tailor our therapeutic approaches to protect the brain from hypoglycemic injury, ultimately permitting tighter glycemic control in diabetes.